Conventionally, vehicle parts have been manufactured through the press forming method.
The method for manufacturing vehicle parts through the press forming method generally manufactures a product through a forming process and a finishing process. The forming process may include cutting the raw material of a cold or hot rolled coil, transferring the cut material into a press die, and pressing the material in the press die, and the finishing process may include trimming unnecessary portions and processing holes.
A steel plate processing method using the conventional press forming method is only to implement the shape of a part by processing the strength and property of a steel plate manufactured by a steel company.
Therefore, the property of the raw material corresponds to the property of the processed part. Thus, when the strength of the raw material is increased, it becomes difficult to perform press forming, thereby making it difficult to implement the precise shape of a product.
Since the recent automobile industry necessarily needs parts which require high strength and formability, the above-described problem of the forming process inevitably limits the application of a raw material with high strength.
The hot stamping method which has been recently introduced is performed at high temperature, while the conventional press forming method is performed at low temperature.
Since the elongation of a material significantly increases at high temperature, the material has no specific limitation in formability, and can achieve a required high-strength characteristic through a heat treatment effect.
Since the hot stamping processes are mostly performed at a high temperature of 900° C. or more, the high-temperature stability of the material serves as an important factor. For this property, a plated steel plate is generally used. For example, an Al—Si-plated steel plate is mainly used as a material used as a hot stamping material. Recently, research has been actively conducted on a variety of plated steel plates with a relatively low price, in addition to the Al—Si-plated steel plate.
FIG. 1 is a flowchart simply illustrating a conventional hot stamping method, and FIG. 2 is a process diagram simply illustrating the conventional hot stamping method.
Referring to FIGS. 1 and 2, the hot stamping method includes a blanking process S-11, a heat treatment process S-12, a hot forming and rapid cooling process S-13 and a trimming process S-14.
The blanking process S-11 is to cut a material into required sizes. The heat treatment process S-12 is to heat the blanked material in a predetermined temperature range. The hot forming and rapid cooling process S-13 is to rapidly cool the heated material while forming the heated material. The trimming process S-14 is to precisely cut the material formed and hardened through the hot forming and rapid cooling process, using laser equipment or the like.
However, when the hot stamping method is performed using the plated steel plate (for example, the Al—Si-plated steel plate), Al plating may stick to the die. The Al plating sticking to the die may cause a defect on the surface of the formed product (or part).
FIG. 3 is a photograph showing that sticking S occurred on the surface of a die 10, when a plated steel plate is used as the material by the conventional hot stamping method.
FIG. 4 is a photograph showing that a plating defect D (also referred to as ‘dent’) has occurred on the surface of the formed product or part 100 after hot stamping, due to the sticking S which occurred on the surface of the die 10 in FIG. 3.
In particular, when the hot stamping method for local softening uses a plated steel plate as the material, Al plating may seriously stick to the surface of the die. In this case, an operator needs to stop the operation of the processing line, check the level of the sticking by checking the surface of the die, and remove the sticking.
The related art is disclosed in Korean Patent Registration No. 10-1033361 registered on Apr. 28, 2011 and entitled “Hot press forming method”.